1. Field of the Invention
The present invention relates to an optical scanner used for laser beam printers, laser facsimiles, digital copying machines, and the like.
2. Description of the Related Art
Many types of optical scanner used for laser beam printers and the like include a semiconductor laser as a light source, a first image formation optical system for converging a light flux from the light source onto an optical deflector linearly for correcting a tilt of a deflection surface of the light deflector, a polygon mirror as the optical deflector, and a second image formation optical system for forming Uniform spots on a surface to be scanned at a constant velocity.
The second image formation optical system is conventionally composed of a plurality of large-size glass lenses called a f.theta. lens. The f.theta. lens is disadvantageous in that it is expensive and the size reduction thereof is difficult. In order to overcome these problems and realize a small-size, inexpensive optical scanner; there have been proposed, for example, optical scanners which use a cylindrical mirror and a cylindrical lens (Japanese Laid-Open Patent Publication No. 1300218), a spherical mirror and a cylindrical lens (Japanese Laid-Open Patent Publication No. 1-300217), and an aspherical mirror and a long cylinder optical element (e.g., a long toric lens) (U.S. Pat. No. 5,408,095) for the second image formation optical system.
However, in any of the above proposed optical systems, the correction of the curvature of field and the f.theta. correction are not sufficient and thus it is difficult to obtain an optical scanner with high resolution.